This invention concerns ink ribbons.
In recent years, impact printers with cost advantages which are capable of high speed printing are finding wide application as man-machine interfaces, for example as peripheral terminal units in data processing systems.
Impact printers which print at high speeds constantly have to deal with a large volume of data, and so it is important that they have reliable print heads. It is moreover highly desirable that print wires operate stably over long periods without suffering corrosion or wear, and without damaging the ink ribbon. These print wires may be made of super-hard or other wear-resistant alloy or of ferrous material which is easy to process and is inexpensive (Patent Application Kokai Publication No. 59-79766).
However, even these wires suffered from the major drawback that when they were used over long periods, the metal constituents of the wire were sometimes chemically corroded.
The corrosion of the wires however also depends on the components of the ink in the ribbon.
The black ink used in conventional ribbons may contain carbon black as coloring material, as disclosed in Patent Application Publication No. 57-60956, and is in fact a mixture of vegetable oil and mineral oil vehicles, carbon black and oil-soluble dyes as coloring materials, and other components such as dispersing agents.
Carbon black normally contains 2-5 weight % ash as impurities, together with sulfur oxides and chloride ions. In the presence of moisture and oxygen in the atmosphere, these impurities cause chemical corrosion of the metal components of the print wire surface, and lead to serious damage such as wire tip wear and wire breakage.
In order to solve these problems, carbon black containing no more than 1% of impurities was used, or the impurities in the carbon black were eliminated in the process of manufacture of the ink ribbon. Pure carbon black is however very costly. Moreover, the elimination of impurities during manufacture of the ink ribbon led to an increase in the number of manufacturing steps, and so the cost of manufacturing the ribbon was again increased.
Another critical factor in high speed printing is that the print head and other moving parts should be lightweight, as is disclosed in for example the Technical Paper of the Institute of Electronics and Communications Engineers of Japan EMC84-2, pp. 9-16. The print wires of super-hard alloy mentioned above contain about 70-85 parts by weight of tungsten carbide, and their density attains 13.5-14.5 g/cm.sup.3. It is thus difficult to make these wires lightweight.
To realize high speed printing, therefore, ordinary ferrous printing wires with a density of approximately 8 g/cm.sup.3 have to be used. These ferrous wires are however not so reliable, as they easily wear down and the life of the print head is short.
The wearing of the print wires is actually a mechanical abrasion due to the ink ribbon. For example, the carbon black contained in the black ink in the conventional ribbons, as is disclosed in the above-mentioned Japanese Patent Application Publication No. 57-60956 has the same effect as minute particles of polishing powder, and in effect causes mechanical wear or "abrasion" of the print wire surface layer.
Instead of carbon black, some ribbon inks use organic pigments to avoid this abrasive wear. However, the print density with respect to near infra-red radiation (wavelength 780-1500 nm) of a print sample produced by these inks, is weaker than that of a sample produced by inks containing carbon black, and problems therefore occurred due to errors when reading the print with an OCR (optical character reader). The life of the ink ribbon was naturally shorter, and the greater length of ribbon necessary to compensate for it led to higher cost. In addition, the printer ribbon cartridge was larger, so that the printer as a whole had to be made bigger.